In a radio communication system, Rayleigh fading occurs with the movement of mobile stations. This Rayleigh fading gives rise to amplitude fluctuation and phase fluctuation in the communication channels. As a result, in a radio communication system such as WCDMA, the transmitting side transmits an already known signal string (pilot signal) on the receiving side. The receiving side estimates the phase fluctuation and amplitude fluctuation of the propagation path based on the differences between the received points of the pilot signal and the already known transmission points (channel estimation). Based on this channel estimation, the receiving side then corrects the fluctuation of the phase and amplitude of an information signal that contains a control signal or data signal (channel elimination).
FIG. 1 shows the use of estimated channels to eliminate the influence of channels from an information signal that has been received in communication that uses QPSK modulation. In phase modulation such as QPSK, phase fluctuation θ that results from the influence of channels is found, and the reception point (R) is moved back by θ to find the reception point (R′) from which the influence of channels has been removed. Generally, in a WCDMA system, higher transmission speeds call for higher accuracy in channel estimation. In WCDMA, the interference component is reduced by the spreading of signals. In high-speed transmission, however, the spreading rate is set low, and the rate of reduction of the interference component is therefore also low.
FIG. 2A shows channel elimination in the case of a high interference component (high-speed transmission). FIG. 2B shows channel elimination in the case of a low interference component (low-speed transmission). When the interference component is high, the spreading of signal points is great, and the circle of distribution of reception points shown in the figure expands. When channel estimation value θ error having low estimation accuracy is used to eliminate channels, the region in which reception error occurs shown by the darkened portion in the figure increases as the interference component increases. Accordingly, in high-speed transmission, a lower rate of reduction of the interference component brought about by spreading calls for greater accuracy in channel estimation values.
Alternatively, in a cellular system that uses direct-sequence spread-spectrum code division multiple access (DS-CDMA) such as a WCDMA system, the same frequency band is used on a plurality of channels, and electromagnetic waves of other channels therefore cause interference. When interference increases, the reception quality of the desired wave deteriorates, causing problems such as circuit disconnections. Accordingly, the number of circuits in which desired reception quality is maintained to realize communication, i.e., the circuit capacity, depends on the amount of interference. In an uplink, the power of a signal that is transmitted by a mobile station remote from a base station undergoes more attenuation than a signal that is transmitted by a mobile station close to the base station. As a result, when these mobile stations transmit signals at the same power, the problem of distance occurs in which the reception power of interference waves from the near mobile station is greater than the reception power of the desired wave from the remote mobile station, and communication with the remote mobile station therefore becomes problematic.
Accordingly, transmission power control is an essential technology on uplinks for controlling the transmission power of each mobile station such that the signal from each mobile station has equivalent reception power at the base station. The base station controls the transmission of mobile stations to realize the minimum necessary transmission power that can maintain the reception quality (reception power-to-interference power ratio or SIR) at a required quality (target SIR). The transmission power control for each mobile station is closed-loop control. The base station compares the measured SIR with a prescribed target SIR, and transmits to a mobile station a transmission power control (TPC) signal instructing that the transmission power be lowered when the measured SIR is higher than the target SIR. Alternatively, when the measured SIR is lower than the target SIR, the base station transmits to the mobile station a TPC signal instructing that the transmission power be raised. By implementing this type of closed-loop control for each slot, the transmission power follows high-speed propagation path fluctuation.
Individual channels for performing communication by circuit switching and EUDCH for high-speed packet transmission are provided in WCDMA uplink channels (Refer to: 3GPPTR25.896 v6.0.0 (2004-03) Third Generation Partnership Project; Technical Specification Group Radio Access Network; Feasibility Study for Enhanced Uplink for UTRA FDD (Release 6)). The individual channels are composed of a DPDCH (Dedicated Physical Data Channel) for transmitting data and a DPCCH (Dedicated Physical Control Channel) for transmitting control signals. The EUDCH is similarly composed of E-DPDCH for transmitting data and E-DPCCH for transmitting control signals. One frame of each channel is composed of 15 slots. Data blocks are transmitted for each prescribed transmission time interval (TTI) by means of each channel. For the TTI of individual channels, any of 1, 2, 4, or 8 frames are used. The TTI of EUDCH are unfixed, but the use of either one frame or ⅕ frame (1 subframe) that is shorter than one frame is predetermined.
FIG. 3 shows the frame configuration of an individual channel. DPCCH contains: the pilot signal, TFCI, FBI, and TPC bits. The pilot signal is used in the above-described channel estimation and SIR measurement. TFCI is a 30-bit control signal for reporting the DPCCH transmission format (data block size and number of blocks) in each TTI; and is transmitted divided into two bits for each slot. Accordingly, after receiving all of one frame, the base station collects and decodes the TFCI that have been divided among the slots and then uses these decoded TFCI to decode DPDCH.
The FBI bits are a field for sending feedback signals that are necessary for other functions in the downlink circuit. TPC is a field for sending the above-described high-speed closed-loop transmission power control signals. In WCDMA, the uplink and downlink of individual channels are paired and are used mutually in the transfer of the transmission power control signals. The EUDCH channels are transmitted at a power obtained by adding an offset power to the individual channels. The EUDCH frame configuration is not yet determined in 3GPP, but TFCI (E-TFCI) of EUDCH exists as a signal transmitted by E-DPCCH. This TFCI is for notifying the transmission format of E-DPCH in this TTI, as with individual channels.
Pilot signals are necessary for channel estimation, but the pilot signals of individual channels can be used in channel estimation. In addition, the base station carries out scheduling such that the noise rise (received signal power to noise ratio) at the base station is no greater than a prescribed target value, and reports the radio resources assigned to mobile stations by EUDCH. Broadly defined, two methods have been investigated for scheduling. One method is referred to as time-transmission speed scheduling, and the other is referred to as transmission speed scheduling.
In time-transmission speed scheduling, the base station designates the transmission time interval and maximum transmission speed by means of scheduling information for each mobile station in which EUDCH is set. The mobile station transmits data blocks at no greater than the designated maximum transmission speed within the designated transmission time interval.
In transmission speed scheduling, on the other hand, the base station designates only the maximum transmission speed by means of the scheduling information. The mobile station may transmit data blocks at any timing as long as the transmission speed is no greater than the maximum transmission speed. The scheduling information can be transmitted for each TTI.
As described above, high-speed packet transmission is carried out in EUDCH, and the transmission speed can be changed in TTI units. As previously described, the accuracy called for in channel estimation increases with higher transmission speeds.
In addition, a high transmission speed calls for high accuracy in the SIR measurement. This is because, since SIR is used in high-speed closed-loop transmission power control, low accuracy in SIR measurement results in degradation of the accuracy of power control, whereby the reception quality of pilot signals deteriorates, causing channel estimation accuracy to also deteriorate. As a result, high transmission speed necessitates greater accuracy in SIR measurement.
Generally, the accuracy of channel estimation and SIR measurement can be improved by either increasing the power of pilot signals with respect to data signals or by increasing the number of pilot bits within each slot. However, these methods increase the control signal overhead and increase the interference upon other mobile stations, and as a result, the constant implementation of these methods is usually not preferable. Accordingly, in EUDCH, improvement of the channel estimation accuracy and SIR measurement accuracy is necessary only in frames that carry out high-speed transmission.
This measure was proposed in “[E]-SPICH Multiplexing Options,” QUALCOMM, 3GPP RAN WG1, 33rd Conference, RI-030673. According to this proposal, in frames in which a mobile station performs high-speed transmission, the transmission of second pilot signals by E-DPCCH is proposed. In this method, a prescribed transmission speed threshold value is determined, and the mobile station transmits second pilot signals on E-DPCCH if the transmission speed is equal to or greater than the threshold value. The base station decodes the E-TFCI and determines that the mobile station is transmitting the second pilot signals if the transmission speed is equal to or greater than the threshold value. In this case, the base station uses the second pilot signals together with the DPCCH pilot signals (first pilot signals) in channel estimation. If the transmission speed is lower than the threshold value, the mobile station does not transmit the second pilot signals, and as a result, the interference that is applied to other mobile stations is reduced when high-accuracy channel estimation is not necessary.